Light-emitting diode incorporating gradient index element

ABSTRACT

The light-emitting device includes a light source and a gradient index (GRIN) element. The GRIN element has a cylindrical refractive index profile in which the refractive index varies radially and is substantially constant axially. The GRIN element includes a first end surface opposite a second end surface and is characterized by a length-to-pitch ratio. The GRIN element is arranged with the first end surface adjacent the light source to receive light from the light source, and emits the light from the second end surface in a radiation pattern dependent on the length-to-pitch ratio. Since the radiation pattern depends on the length-to-pitch ratio of the GRIN element, LEDs with different radiation patterns can be made simply by using GRIN elements of appropriate lengths.

BACKGROUND OF THE INVENTION

Light-emitting diodes (LEDs) have a wide range of applications inindustry. An LED is typically composed of a semiconductor die mounted ona header that provides mechanical support for and electrical connectionsto the semiconductor die. The semiconductor die is then encapsulated toprotect it. The encapsulation is transparent or includes a transparentwindow to allow light generated by the semiconductor die to be emitted.The header and the encapsulation collectively constitute the package ofthe LED.

Parameters used to characterize the performance of an LED includebrightness and radiant power. Another important parameter used tocharacterize the performance of an LED is the radiation pattern, i.e.,the dependence of brightness on angle relative to the optical axis ofthe LED. The semiconductor die emits light in a substantially isotropicradiation pattern, which is suitable for relatively few applications.Typical LEDs include a converging element that includes a convex surfaceto generate a specific radiation pattern from the isotropic radiationpattern of the semiconductor die. Examples of such converging elementsinclude a convex spherical or aspherical lens and encapsulation thatincludes a convex surface facing the semiconductor die.

Different applications use LEDs with different radiation patterns. Forexample, an LED configured for use in back lighting for an LCD screenhas a relatively wide and uniformly-spread radiation pattern, whereas anLED configured for use in a signal light, such as a traffic light, has arelatively narrow radiation pattern and a high intensity. An LEDconfigured for use in a position or rotation encoder has a collimated orpoint source radiation pattern.

The need to provide LEDs configured for many different applications thatrequire different radiation patterns cause manufacturers of LEDs to usea large number of different types of LED package, each with its ownshape. The need to keep an inventory of many types of package to meetcustomers' demands contributes to the manufacturing cost of LEDs. Sincemanufacturers are under constant pressure to reduce costs, what isneeded is an LED that can easily be manufactured with a range ofdifferent radiation patterns without the need to keep an inventory ofmany different types of package. What is also needed is a way tomanufacture such LEDs.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a light-emitting device thatincludes a light source and a gradient index (GRIN) element. The GRINelement has a cylindrical refractive index profile in which therefractive index varies radially and is substantially constant axially.The GRIN element includes a first end surface opposite a second endsurface and is characterized by a length-to-pitch ratio. The GRINelement is arranged with the first end surface adjacent the light sourceto receive light from the light source. The GRIN element emits the lightfrom the second end surface in a radiation pattern dependent on thelength-to-pitch ratio.

In a second aspect, the invention provides a method of making alight-emitting device. In the method, a light source and a GRIN elementare provided. The GRIN element has a cylindrical refractive indexprofile in which the refractive index varies radially and issubstantially constant axially. The GRIN element includes a first endsurface opposite a second end surface and is characterized by alength-to-pitch ratio. Additionally in the method, the GRIN element isarranged with the first end surface thereof adjacent the light source toreceive light from the light source. The GRIN element emits the lightfrom the second end surface in a radiation pattern that depends on thelength-to-pitch ratio.

Since the radiation pattern depends on the length-to-pitch ratio of theGRIN element, LEDs with different radiation patterns can be made simplyby using GRIN elements of appropriate lengths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are an end view and a side view, respectively, of anembodiment of a light-emitting device (LED) in accordance with theinvention.

FIGS. 2A and 2B are graphs showing an example of how the cylindricalrefractive index profile of a GRIN element respectively varies radiallyand axially.

FIG. 2C is a graph showing an example of an alternative radialvariation.

FIGS. 3A–3D schematically show the radiation patterns of four exemplaryembodiments of an LED in accordance with the invention with differentlength-to-pitch ratios.

FIGS. 4A–4D are respectively a side view, a top view, a cross-sectionalview and an isometric view of another embodiment of an LED in accordancewith the invention. The cross-sectional view of FIG. 4C is taken alongthe section line 4C—4C in FIG. 4B.

FIGS. 5A–5C are respectively an exploded side view, a top view and across-sectional view of another embodiment of an LED in accordance withthe invention. The cross-sectional view of FIG. 5C is taken along thesection line 5C—5C in FIG. 5B.

FIGS. 6A–6C are respectively an exploded side view, a top view and across-sectional view of another embodiment of an LED in accordance withthe invention. The cross-sectional view of FIG. 6C is taken along thesection line 6C—6C in FIG. 6B.

FIG. 6D is an end view of the GRIN element of the LED shown in FIGS.6A–6C.

FIGS. 7A–7C are respectively an exploded side view, a top view and across-sectional view of another embodiment of an LED in accordance withthe invention. The cross-sectional view of FIG. 7C is taken along thesection line 7C—7C in FIG. 7B.

FIGS. 8A–8C are respectively an exploded side view, a top view and across-sectional view of another embodiment of an LED in accordance withthe invention. The cross-sectional view of FIG. 8C is taken along thesection line 8C—8C in FIG. 8B.

FIG. 9 is a cross-sectional view of another embodiment of an LED inaccordance with the invention. The cross section is taken along asection line similar to the section line 8C—8C in FIG. 8B.

DETAILED DESCRIPTION OF THE INVENTION

Light is refracted when it passes through an interface between materialsof different refractive index. The refraction follows the well-knownSnell's law. Many conventional optical systems incorporate lenses andother elements with at least one curved surface through which light ispassed to perform a desired function such as focusing or collimating.Such refracting elements and the media in which they are locatedtypically have a homogeneous refractive index.

Gradient index (GRIN) material has become affordable in the past decade.Such material has a non-homogeneous refractive index. Optical deviceswith many of the optical properties of a conventional lens can be madeusing GRIN material having flat surfaces instead of the curved surfacesof a conventional lens. One type of GRIN material has a cylindricalrefractive index profile in which the refractive index varies radiallyand in which the refractive index at a given radius is substantiallyconstant axially. The refractive index varies radially in the sense thatit decreases with increasing distance from the axis of radial symmetryof the cylindrical refractive index profile. A GRIN element having acylindrical refractive index profile is typically cylindrical in shape,but is not necessarily cylindrical in shape.

FIGS. 1A and 1B are an end view and a side view, respectively, of afirst embodiment 10 of a light-emitting device (LED) in accordance withthe invention. LED 10 is composed of a GRIN element 12 and alight-emitting semiconductor die 14 located adjacent an end surface 16of the GRIN element. Light generated by semiconductor die 14 passesaxially through GRIN element 12 and is emitted from end surface 18,opposite end surface 16. Both end surfaces 16 and 18 are substantiallyplane surfaces.

GRIN element 12 has a cylindrical refractive index profile. In theexample shown, GRIN element 12 is also cylindrical in shape, but othershapes are possible.

FIGS. 2A and 2B show an example of how the cylindrical refractive indexprofile of GRIN element 12 respectively varies radially and axially.FIG. 2C shows an example of an alternative radial variation. FIGS. 2Aand 2C show examples of how the cylindrical refractive index profilesvaries radially in a plane 22 orthogonal to the axis of radial symmetry20 of the cylindrical refractive index profile. Plane 22 is located inGRIN element 12 at an arbitrary distance z₁ from end surface 16. Theorigins in FIGS. 2A and 2C correspond to axis 20. FIG. 2B shows anexample of how the cylindrical refractive index profile of GRIN element12 varies axially at a radius r₁ from the axis of radial symmetry of thecylindrical refractive index profile. The origin in FIG. 2B correspondsto end surface 16. FIG. 2A shows an example in which the refractiveindex decreases smoothly with increasing radius from axis 20. FIG. 2Cshows an example in which the refractive index decreases in steps withincreasing radius from axis 20.

FIG. 2A shows that, in plane 22, the refractive index n_(r) at radius rprogressively decreases with increasing radius from axis 20. FIG. 2Cshows that, in plane 22, the refractive index n_(r) at radius rdecreases in steps with increasing radius from axis 20. The variation ofrefractive index with radius in a plane orthogonal to axis 20 would besimilar at any point along axis 20 to that shown in FIG. 2A or FIG. 2C.Moreover, the refractive index at radius r₁ would nominally be the sameregardless of the angle θ between an arbitrary reference direction andthe direction in which the radius r is measured.

FIG. 2B shows that the refractive index n_(z) at distance z and radiusr₁ remains constant regardless of the distance z from end surface 16.The refractive index at distance z also remains nominally constantregardless of distance at any other radius and at any value of the angleθ between the direction in which the radius r is measured and anarbitrary reference direction. In practical GRIN elements, smallvariations in the nominal refractive index with distance z occur.

In an exemplary embodiment, the variation of refractive index n_(r) withradius r of the cylindrical refractive index profile is described byequation (1):n _(r) =n ₀(1−(Ar ²)/2)  (1)where n_(r) is the refractive index at radius r, n₀ is the refractiveindex at the center of the cylindrical refractive index profile, i.e.,at r=0, and A is a material-dependent gradient constant. In thisexample, the variation of refractive index with radius has a quadraticfunction, with the steepness of the change of refractive index beingdetermined by the value of the gradient constant A.

FIG. 1B shows the path 24 of light emitted by semiconductor die 14 in adirection non-parallel to axis of radial symmetry 20. Path 24 bends asthe light travels from end surface 16 towards end surface 18. The pathbends initially due to the light encountering a progressively decreasingrefractive index. As a result of the bending of path 24, the anglebetween the path and axis 20 progressively decreases with increasingdistance from end surface 16. At a certain distance from end surface 16,path 24 is parallel to axis 20. At distances greater than this certaindistance, path 24 bends towards axis 20 as the light encounters aprogressively increasing refractive index. Eventually, path 24re-crosses axis 20 as shown. After re-crossing the axis, path 24 bendstowards the axis, becomes parallel to the axis and bends away from theaxis until it re-crosses the axis a second time. The second re-crossingof the axis is in the same direction as the initial direction of path 24adjacent end surface 16. The path shape just described repeats along thelength l of GRIN element 12 until the light reaches end surface 18. Thelight is then emitted from end surface 18.

The bending of the path 24 of the light emitted by semiconductor die 14prevents the light from escaping from the curved surface 26 of GRINelement 12. Instead, most of the light emitted by semiconductor die 14is directed towards end surface 18, from which it is emitted. As thelight travels further in the z-direction from end surface 16, GRINelement 12 repetitively re-focuses light at respective focal pointsoffset in the z-direction from end surface 16.

GRIN element 12 is characterized by a parameter called pitch, which isthe distance light travels in the z-direction to path 24 undergo onefull sinusoidal cycle in which the light diverges from source, convergeson a first focal point, diverges from the first focal point andconverges on a second focal point. The path of the light passes throughthe second focal point in the same direction as initial direction of thepath adjacent end surface 16. The pitch of the GRIN element depends onthe above-mentioned gradient constant A. The relationship between thelength of the GRIN element, the gradient constant A and the pitch p isgiven by equation (2):p=(√A/2π)z  (2)

End surface 18 is a plane surface. Light travelling through GRIN element12 and incident on end surface 18 at a non-zero angle of incidence isrefracted at end surface 18. In air, the angle of refraction is greaterthan the angle of incidence in accordance with Snell's law. It can beseen from FIG. 1B that the angle of incidence of light on end surface 18depends on the distance of end surface 18 from end surface 16, i.e., onthe physical length of GRIN element 12. Accordingly, the radiationpattern of the light emitted from end surface 18 depends on theradiation pattern of the light incident on end surface 18. The radiationpattern of the light incident on end surface 18 depends on the distanceof end surface 18 from end surface 16. Therefore, the radiation patternof the light emitted from end surface 18 of LED 10 will depend on thelength-to-pitch ratio of GRIN element 12.

Thus, the invention provides LEDs with configurable radiation patterns.The radiation pattern of each LED is defined simply by appropriatelyselecting the length-to-pitch ratio of the GRIN element 12. The LEDmanufacturer no longer has to keep a large inventory of dome lenses ordome lens molds to be able to manufacture LEDs with different radiationpatterns. Moreover, the invention allows the radiation pattern of anexisting GRIN element-based LED to be changed simply by changing thephysical length of the GRIN element.

FIGS. 3A–3D schematically show the radiation patterns of four exemplaryembodiments of LED 10 with different length-to-pitch ratios. Theradiation patterns are characterized by an angle α, which is the anglebetween the axis of symmetry of the cylindrical refractive index profileof the GRIN element and the direction at which the intensity of thelight emitted by the LED falls to one-half of maximum.

In exemplary embodiment 30 shown in FIG. 3A, GRIN element 32 has aphysical length equal to one half of the pitch, i.e., l=p/2, and alength-to-pitch ratio of 0.5. GRIN element 32 focuses the light emittedby semiconductor die 14 located adjacent end surface 36 on end surface38. Accordingly, LED 30 emits the light from a point source. Practicalembodiments of LED 30 do not emit the light from an ideal point sourcebecause the light-emitting area of semiconductor die 14 is notinfinitesimally small. The term from a point source as used in thisdisclosure will be taken to encompass such a non-ideal point source.

In embodiment 40 shown in FIG. 3B, GRIN element 42 has a physical lengthintermediate between one quarter and one half of the pitch, i.e.,p/4<l<p/2, and a length-to-pitch ratio between 0.25 and 0.5. The lightemitted by semiconductor die 14 located adjacent end surface 46 isconverging when it reaches end surface 48 after passing through GRINelement 42. After refraction by passing through end surface 48, thelight emitted by LED 40 has a converging radiation pattern characterizedby the angle α.

In embodiment 50 shown in FIG. 3C, GRIN element 52 has a physical lengthequal to one quarter of the pitch, i.e., l=p/4, length-to-pitch ratio of0.25. The light emitted by semiconductor die 14 located adjacent endsurface 56 is traveling orthogonal to end surface 58 when reaches endsurface 58 after passing through GRIN element 52. After passing throughend surface 58, the light is emitted by LED 50 in a parallel radiationpattern characterized by angle α of zero. The light emitted byembodiment 50 is a collimated beam.

In embodiment 60 shown in FIG. 3D, GRIN element 62 has a physical lengthless than one quarter of the pitch, i.e., l<p/4, and a length-to-pitchratio of less than 0.25. The light emitted by semiconductor die 14located adjacent end surface 66 is diverging when it reaches end surface68 after passing through GRIN element 62. After refraction by passingthrough end surface 68, the light emitted by LED 60 has a divergingradiation pattern characterized by the angle α.

FIGS. 4A–4D are respectively a side view, a top view, a cross-sectionalview and an isometric view of another embodiment 100 of an LED inaccordance with the invention. LED 100 is composed of a GRIN element110, a semiconductor die 120 and a cup-type header 130.

Header 130 has a device mounting surface 132 and a surface 134 oppositethe device mounting surface. LED 100 is mounted on a printed circuitboard or other substrate (not shown) with device mounting surface 132facing the printed circuit board. Device mounting surface 132 carriesconductive elements (not shown to simplify the drawing) electricallyconnected to semiconductor die 120.

Header 130 defines two concatenated cavities 140 and 142 that extend, inorder, into the header from header surface 134. Cavity 140 iscylindrical in shape. Cavity 140 is a partially closed cavity and has anannular, flat end surface 144 remote from header surface 134 and acurved side surface 146. Cavity 142 is cylindrical in shape with asmaller diameter than cavity 140, but can have other shapes. Cavity 142is a closed cavity and has a flat die mounting surface 148 remote fromheader surface 134. Cavity 142 also has a curved side surface 150.

Cavity 142 accommodates semiconductor die 120 mounted on the diemounting surface 148. Cavity 142 additionally accommodates an electricalconnection, typically a wire bond, that extends between the exposedsurface of the die and an electrical conductor that extends throughheader 130 to mounting surface 132. Neither the electrical connectionnor the electrical conductor is shown to simplify the drawing.

Cavity 142 additionally accommodates index matching material 154 havinga refractive index greater than that of air and preferably intermediatebetween those of semiconductor die 120 (typically greater than 3) andGRIN element 110 (typically about 1.5). Suitable index matchingmaterials include optical gels and optical liquids sold by CargilleLaboratories of Cedar Grove, N.J. Another suitable material is anoptical adhesive composed of Ablebond® epoxy resin or HXTAL NYL-1 epoxyresin and a mixture of 1,2-epoxy-3-[2,4,6-tribromophenoxy] propane and1,2-epoxy-3-[2,4,6-triiodophenoxy] propane. This adhesive is describedin 28–2 J. AM. INST. CONSERVATION. 127–136 (1989).

GRIN element 110 has a body 112 and exhibits the cylindrical refractiveindex profile described above. Body 112 is cylindrical in shape. Theaxes of symmetry of the refractive index profile and the body coincide.Body 112 has a curved side surface 114 and opposed end surfaces 116 and118. End surface 116 is flat. In the example shown, end surface 118 isalso flat. Both end surfaces are orthogonal to the axes of symmetry ofbody 112 and its cylindrical refractive index profile. In otherembodiments, at least end surface 118 is curved, e.g., convex, but, inaccordance with the invention, LEDs having different radiation patternscan all be made each with a body 112 of a different length and an endsurface 118 of the same shape.

The length of body 112 depends on the pitch of the cylindricalrefractive index profile of body 112 and the desired radiation pattern.As noted above, LEDs having many different radiation patterns can all bestructurally identical apart from the length-to-pitch ratio of theirGRIN elements 110.

The diameter of cylindrical cavity 140 defined in header 130 iscomparable with the diameter of body 112. In one embodiment, thediameter of cavity 140 is slightly less than that of body 112, whichmakes GRIN element 110 a push fit into cavity 140. A diameter differencein the range of 30–40 μm is sufficient to allow the GRIN element to bepushed into cavity 140 during assembly yet allows the header to retainGRIN element in place throughout the operational life and environmentalrange of LED 100. The mutual deformation of header 130 and GRIN element110 resulting from the push fit forms a hermetic seal between the GRINelement and the header. Thus, GRIN element 110 and header 130collectively encapsulate semiconductor die 120, and no additionalelement is needed for this purpose. End surface 144 of cavity 140defines the extent to which GRIN element fits into the header.

In other embodiments, the diameter of cavity 140 is greater than that ofbody 112 and GRIN element 110 is a loose fit in cavity 140. In suchembodiments, the GRIN element is retained in the cavity 140 of header130 by a film of a suitable adhesive that fills the gap between the sidesurface 146 of cavity 140 and the curved surface 114 of body 112. GRINelement 110, header 130 and the adhesive collectively encapsulatesemiconductor die 120. Other ways of retaining GRIN element 110 inheader 130 and of encapsulating semiconductor die 120 may be used.

GRIN element 110 is inserted into cavity 140 until end surface 116 ofbody 112 abuts the end surface 144 of cavity 140. The juxtaposition ofend surfaces 116 and 144 defines the spatial relationship between thelight-emitting surface of semiconductor die 120 and end surface 116 ofGRIN element 110. This relationship is defined regardless of the way inwhich GRIN element 110 is retained in header 130.

LED 100 is made by mounting semiconductor die 120 on die mountingsurface 148 of cavity 142 in header 130. An electrical connection (notshown) is between an electrode (not shown) typically located on thelight-emitting surface of the semiconductor die and a conductor (notshown) that extends through the header to a conductive element (notshown) on device mounting surface 134. Processes for mountingsemiconductor die on headers and for making electrical connectionsbetween semiconductor die and headers are known in the art and willtherefore not be described here. Preliminary electrical andelectro-optical testing may be carried out at this stage.

A measured quantity of index matching material 154 is introduced intocavity 142, specifically onto the light-emitting surface ofsemiconductor die 120. In one embodiment, part of the index-matchingmaterial projects into cavity 140 to ensure that the index material willcontact the end surface 116 of GRIN element 110 when the latter isinserted into cavity 140.

GRIN element 110 and header 130 are then assembled as described above.LED 100 is then ready for final electrical and electro-optical testing.

In a process in which LEDs 100 are mass manufactured, hundreds ofheader-die assemblies, i.e., semiconductor die 120 mounted in header130, are loaded into a first jig and a corresponding number of GRINelements 110 are loaded into a second jig having the same pitch as thefirst jig. The jigs are placed in a suitable press with the GRINelements facing the cavities 140 in the headers 130. The press is thenused to press all the GRIN elements into the cavities 140 in therespective headers.

GRIN material is available in the form of cylindrical rods having alength many times that of the individual GRIN elements. The GRINmaterial is typically composed of a rod of a host material such assilica (SiO₂) doped with an index-changing dopant. The dopant isdeposited on the curved surface of the rod of host material. The rodwith the dopant on its curved surface is then heated and the dopantdiffuses radially towards the center of the rod. Dopants such as boronor fluorine reduce the refractive index of the host material to producea cylindrical refractive index profile similar to that shown in FIGS. 2Aand 2B in which the refractive index decreases with increasing radius.

Another embodiment is made from materials composed of nanoparticles ofone or more transparent high refractive index metal oxides such astitanium dioxide (TiO₂), magnesium oxide (MgO), zirconium oxide (ZrO₂)and aluminum oxide (Al₂O₃) embedded in a carrier of a low refractiveindex material such as epoxy. The density of the nanoparticlesdetermines the refractive index of the material. A rod of a highrefractive index form of the material, or another high refractive indexmaterial, is successively coated with layers of progressively lowerrefractive index forms of the material to form a rod of GRIN materialhaving a cylindrical refractive index profile in which the refractiveindex decreases step-wise with increasing radius, as shown in FIG. 2C.

In another embodiment, a filament of an index-increasing dopant materialis coated with a transparent host material. The resulting assembly isheated, which causes the index-increasing dopant material to diffuseradially outwards and produce in the host material a cylindricalrefractive index profile in which the refractive index decreasesprogressively with increasing radius. This process establishes acylindrical refractive index profile in the host material regardless ofthe cross-sectional shape of the host material. Thus, the host materialneed not be cylindrical in shape. The techniques exemplified above andother techniques for making rods of GRIN material are known in the artand will therefore not be described in more detail here.

Multiple GRIN elements similar to GRIN element 110 are made by dividinga cylindrical rod of GRIN material into multiple cylindrical pieces eachof a length that, in relation to the pitch of the GRIN material, gives alength-to-pitch ratio corresponding to the desired radiation pattern.The pieces into which a rod of GRIN material is divided are typicallyall of the same length. When LEDs having a variety of radiation patternsare being produced, the cylindrical pieces into which a rod of GRINmaterial is divided can have a number of different lengths correspondingto the mix of radiation patterns. The mix of lengths into which a rod ofGRIN material is divided can be determined using a yield maximizationprocess to reduce wastage.

The rod of GRIN material is divided into cylindrical pieces by processessuch as cleaving, cutting and sawing. Techniques, such as hot cuttingand laser cutting, used for cutting optical fibers are also applicableto divide the rod of GRIN material into GRIN elements 110. Some dividingprocess produce cylindrical pieces that are immediately ready forinstallation in respective headers as GRIN elements. Such cylindricalpieces are produced with a length equal to the length that gives thelength-to-pitch ratio corresponding to the desired radiation pattern.

The cylindrical pieces produced by other dividing processes have roughend surfaces that require polishing to produce a respective GRINelement. Cylindrical pieces subject to polishing initially have a lengthslightly longer than the length that gives the length-to-pitch ratiocorresponding to the desired radiation pattern. The end surfaces of suchcylindrical pieces are polished using chemical-mechanical polishing(CMP), for example, or another suitable process to form respective GRINelements. Polishing reduces the length of the cylindrical pieces to thelength corresponding to the desired radiation pattern. The purpose ofpolishing after dividing the rod of GRIN material is to make sure theend surfaces are flat and are free of undesired material that woulddegrade the optical performance. High-performance polishing is generallynot needed. The resulting GRIN elements are then ready for installationin respective headers.

In applications in which LEDs with accurately defined radiation patternsare needed, the pitch of each rod of GRIN material is measured and thelengths that, together with the measured pitch, give the length-to-pitchratio corresponding to the desired radiation patterns are calculated.The rod is then divided into cylindrical pieces having the calculatedlengths.

The simplicity of GRIN element 110 makes the process for manufacturingGRIN elements very flexible so that changes in the mix of radiationpatterns demanded by the market can be responded to without the need tocarry a large inventory of different parts. Moreover, excess inventoryof, for example, finished point source LEDs (FIG. 3A) can be convertedinto LEDs having another radiation pattern for which there is marketdemand by dividing the GRIN elements of the LEDs lengthways to reducetheir length to provide a length-to-pitch ratio corresponding to theneeded radiation pattern.

FIGS. 5A–5C are respectively an exploded side view, a top view and across-sectional view of another embodiment 200 of an LED in accordancewith the invention. LED 200 is composed of a GRIN element 210,semiconductor die 120 and a cup-type header 230.

Header 230 has a device mounting surface 232 and a surface 234 oppositethe device mounting surface. LED 200 is mounted on a printed circuitboard or other substrate (not shown) with device mounting surface 232facing the printed circuit board. Device mounting surface 232 carriesconductive elements (not shown to simplify the drawing) electricallyconnected to semiconductor die 120.

Header 230 defines two concatenated cylindrical cavities 240 and 242that extend, in order, into the header from header surface 234. Cavity240 is cylindrical in shape. Cavity 240 is a partially closed cavity andhas an annular, flat end surface 244 remote from header surface 234 anda threaded, curved side surface 246. Cavity 242 is cylindrical in shapewith a smaller diameter than cavity 240, but can have other shapes.Cavity 242 is a closed cavity and has a flat die mounting surface 248remote from header surface 234. Cavity 242 also has a curved sidesurface 250.

Cavity 242 accommodates semiconductor die 120 mounted on die mountingsurface 248. Cavity 242 additionally accommodates above-described indexmatching material 154 and an electrical connection, typically a wirebond, that extends between the exposed surface of the die and anelectrical conductor that extends through header 230 to mounting surface232. Neither the electrical connection nor the electrical conductor areshown to simplify the drawing.

GRIN element 210 has a body 212 that exhibits the cylindrical refractiveindex profile described above. Body 212 is cylindrical in shape. Theaxes of symmetry of the refractive index profile and the body coincide.Body 212 has a curved side surface 214 and opposed end surfaces 216 and218. End surface 216 is flat. In the example shown, end surface 218 isalso flat. Both end surfaces are orthogonal to the axes of symmetry ofbody 112 and its cylindrical refractive index profile. In otherembodiments, at least end surface 218 is curved, as described above.Body 212 may have shapes other than cylindrical, but exhibits acylindrical refractive index profile regardless of its shape.

Body 212 includes a cylindrical, threaded portion 215 adjacent andextending orthogonally to end surface 216. The threaded portion isstructured to engage with the threaded side surface 246 of cavity 240 inheader 230.

The length of body 212 depends on the pitch of the cylindricalrefractive index profile of body 212 and the desired radiation pattern.As noted above, LEDs having many different radiation patterns can all bestructurally identical apart from the length-to-pitch ratio of the body212 of their GRIN elements 210.

LED 200 is made in a manner similar to that described above by mountingsemiconductor die 120 on the die mounting surface 248 of cavity 242 inheader 230, making an electrical connection (not shown) between theheader and the light-emitting surface of the die, and introducing ameasured quantity of index matching material 154 into cavity 242.Threaded portion 215 of GRIN element 210 is then engaged with thethreaded side surface 246 of cavity 240 in header 230 and GRIN element210 is rotated relative to header 230 until the end surface 216 of theGRIN element abuts the end surface 244 of cavity 240. The juxtapositionof end surface 216 and end surface 244 defines the spatial relationshipbetween the light-emitting surface of semiconductor die 120 and the endsurface 216 of GRIN element 210. In one embodiment, an adhesive (notshown) is applied to either or both of threaded portion 215 and threadedside surface 246 prior to engaging the threads. After curing, theadhesive holds GRIN element 210 in place in header 230 and preventscontaminants from leaking past the threads and contaminatingsemiconductor die 120. LED 200 is then ready for final electrical andelectro-optical testing.

GRIN element 210 is manufactured by using a molding process to form acylindrical core that defines the basic shape of body 212. The moldingprocess additionally defines threaded portion 215 of body 212. A dopantis then deposited on curved side surface 214 by a suitable depositionprocess such as chemical vapor deposition (CVD). The core is then heatedto a temperature at which the dopant diffuses into the core. Thediffusion process is continued until the desired cylindrical refractiveindex profile is obtained. In an another process that can be used tofabricate GRIN element 210, a cylindrical core having a threaded portionis formed as described above, and layers of transparent material havingprogressively smaller refractive indices are successively deposited onthe curved side surface of the core to form an embodiment of GRINelement 210 having a stepped refractive index profile similar to thatshown in FIG. 2C. In yet an another process that can be used tofabricate GRIN element 210, a cylindrical core having a threaded portionis formed as described above, and the core is dipped into transparentliquid materials having progressively smaller refractive indices to formthin layers of materials of diminishing refractive index on curved sidesurface of the core. This process also forms an embodiment of GRINelement 210 having a stepped refractive index profile. Suitablematerials are described above. Each layer is allowed to solidify beforethe next layer is deposited.

In some fabrication embodiments, the length of the core used tofabricate body 212 is that which, together with the pitch obtained bydiffusing a dopant into the core or by coating the core, has thelength-to-pitch ratio that produces the desired radiation pattern. Aninventory of GRIN elements of different lengths is kept to allow LEDswith different radiation patterns to be made. Even with fixed-lengthGRIN elements, some variation in the radiation pattern can be obtainedby varying the refractive index profile of the GRIN element while it isbeing made.

The number of different length-to-pitch ratios of GRIN element that needto be kept in stock for use in making LEDs in accordance with theinvention can be reduced by making the GRIN elements longer thannecessary. The GRIN elements are then divided lengthwise aftermanufacture to obtain the length-to-pitch ratio that produces thedesired radiation pattern. GRIN elements of only a singlelength-to-pitch ratio need be stocked if materials wastage is not animportant consideration. Such GRIN elements are those that have alength-to-pitch ratio corresponding to the point source radiationpattern shown in FIG. 3A. GRIN elements that produce other radiationpatterns are made simply by dividing the GRIN element lengthwise toproduce a GRIN element that has a length that gives the length-to-pitchratio corresponding to the desired radiation pattern. Less wastage isproduced by stocking GRIN elements with a number of different lengthsand dividing them lengthwise to obtain the desired length-to-pitchratio.

FIGS. 6A–6C are respectively an exploded side view, a top view and across-sectional view of another embodiment 300 of an LED in accordancewith the invention. FIG. 6D is an end view of the GRIN element of theLED. In this embodiment, the header defines one cylindrical cavity andthe GRIN element defines another cylindrical cavity.

LED 300 is composed of a GRIN element 310, semiconductor die 120 and acup-type header 330.

Header 330 has a device mounting surface 332 and a surface 334 oppositethe device mounting surface. LED 300 is mounted on a printed circuitboard or other substrate (not shown) with device mounting surface 332facing the printed circuit board. Device mounting surface 332 carriesconductive elements (not shown) electrically connected to semiconductordie 120.

Header 330 defines a cylindrical cavity 340 that extends into the headerfrom header surface 334. Cavity 340 is a closed, cylindrical cavity thathas a flat die mounting surface 344 remote from header surface 334.Cavity 340 also has a threaded, curved side surface 346. Semiconductordie 120 is mounted on the die mounting surface 344 of cavity 340.

GRIN element 310 has a body 312 that exhibits the cylindrical refractiveindex profile described above. Body 312 is cylindrical in shape. Theaxes of symmetry of the refractive index profile and the body coincide.Body 312 has a curved side surface 314 and opposed end surfaces 316 and318. End surface 316 is annular, flat and orthogonal to theabove-mentioned axes of symmetry. In the example shown, end surface 318is also flat and orthogonal to the above-mentioned axes of symmetry. Inother embodiments, at least end surface 318 is curved, as describedabove. Body 312 may have shapes other than cylindrical, but exhibits acylindrical refractive index profile regardless of its shape.

Body 312 includes a cylindrical, threaded portion 315 adjacent andextending orthogonally to end surface 316. The threaded portion isstructured to engage with the threaded side surface 346 of cavity 340 inheader 330.

Additionally defined in the body 312 of GRIN element 310 is a cavity 342that extends into the body from end surface 316. Cavity 342 is a closedcavity that has an end surface 348 remote from end surface 316. Endsurface 348 is flat and is orthogonal to the axes of symmetry of body312 and its cylindrical refractive index profile. In the example shown,cavity 342 is cylindrical and has a curved side surface 350. Othercavity shapes are possible.

When LED 300 is assembled, cavity 342 accommodates semiconductor die120, an electrical connection, and above-described index matchingmaterial 154. The electrical connection is typically a wire bond andextends between the exposed surface of the die and an electricalconductor that extends through header 330 to mounting surface 332.Neither the electrical connection nor the electrical conductor is shownto simplify the drawing.

The length of body 312 depends on the pitch of the cylindricalrefractive index profile of body 312 and the desired radiation pattern.As noted above, LEDs having many different radiation patterns can all bestructurally identical apart from the length-to-pitch ratio of the body312 of their GRIN elements 310.

LED 300 is made in a manner similar to that described above by mountingsemiconductor die 120 on the die mounting surface 344 of cavity 340 inheader 330, making an electrical connection between the header and thelight-emitting surface of the die, and introducing a measured quantityof index matching material 154 into cavity 342 in the body 312 of GRINelement 310. Threaded portion 315 of the body 312 of GRIN element 310 isthen engaged with the threaded side surface 346 of cavity 340 in header330 and GRIN element 310 is rotated relative to header 330 until the endsurface 316 of the GRIN element abuts the end surface 344 of cavity 340.The juxtaposition of end surface 316 and end surface 344 defines thespatial relationship between the light-emitting surface of semiconductordie 120 and the end surface 348 of cavity 342 in GRIN element 310. Inone embodiment, an adhesive (not shown) is applied to either or both ofthreaded portion 315 and threaded side surface 346 prior to engaging thethreads. After curing, the adhesive holds GRIN element 310 in place inheader 330 and prevents contaminants from leaking past the threads andcontaminating semiconductor die 120. LED 300 is then ready for finalelectrical and electro-optical testing.

GRIN element 310 is manufactured by processes similar to those describedabove for forming GRIN element 210 with the exception that the moldingprocess that defines the basic shape of the body 312 additionallydefines threaded portion 315 and cavity 342 that extends into body 312from end surface 316. GRIN elements 310 with different length-to-pitchratios can be provided in ways similar to those described above withreference to GRIN element 210.

FIGS. 7A–7C are respectively an exploded side view, a top view and across-sectional view of another embodiment 400 of an LED in accordancewith the invention. In this embodiment, the header defines a cylindricalcavity and the GRIN element has stepped side surface and a threadedportion that is shorter than the depth of the cavity in the header. LED400 is composed of a GRIN element 410, semiconductor die 120 and acup-type header 430.

Header 430 has a device mounting surface 432 and a surface 434 oppositethe device mounting surface. LED 400 is mounted on a printed circuitboard or other substrate (not shown) with device mounting surface 432facing the printed circuit board. Device mounting surface 432 carriesconductive elements (not shown to simplify the drawing) electricallyconnected to semiconductor die 120.

Header 430 defines a cylindrical cavity 440 that extends into the headerfrom header surface 434. Cavity 440 is a closed, cylindrical cavity thathas a flat die mounting surface 444 remote from header surface 434.Cavity 440 also has a threaded, curved side surface 446. Semiconductordie 120 is mounted on the die mounting surface 444 of cavity 440.

GRIN element 410 has a body 412 that exhibits the cylindrical refractiveindex profile described above. Body 412 is cylindrical in shape. Theaxes of symmetry of the refractive index profile and the body coincide.Body 412 has a curved side surface 414 and opposed, flat end surfaces416 and 418. End surfaces 416 and 418 are circular, flat and orthogonalto the above-mentioned axes of symmetry. In other embodiments, at leastend surface 418 is curved, as described above.

Body 412 can be regarded as having two cylindrical body portions 450 and452 concatenated lengthways with body portion 450 adjacent end surface416 and body portion 452 adjacent end surface 418. Body portion 450 issmaller in diameter than body portion 452. Hence, end surface 416 issmaller in diameter than end surface 418. Body portion 450 has athreaded, curved side surface 415 structured to engage with the threadedside surface 446 of cavity 440 in header 430. Body portion 452 has acurved side surface 454 and an annular end surface 456.

The length of body portion 450 of the body 412 of GRIN element 410 isless than the depth of cavity 440 in header 430. Hence, when LED 400 isassembled, body portion 450 does not occupy all of cavity 440. Header430 and body portion 450 of GRIN element 410 collectively define acavity 442, i.e., the portion of cavity 440 not occupied by body portion450. Cavity 442 accommodates semiconductor die 120, an electricalconnection, and above-described index matching material 154. Theelectrical connection is typically a wire bond and extends between theexposed surface of the die and an electrical conductor that extendsthrough header 430 to device mounting surface 432. Neither theelectrical connection nor the electrical conductor is shown to simplifythe drawing.

The length of body 412 depends on the pitch of the cylindricalrefractive index profile of body 412 and the desired radiation pattern.As noted above, LEDs having many different radiation patterns can all bestructurally identical apart from the length-to-pitch ratio of the body412 of their GRIN elements 410.

LED 400 is made in a manner similar to that described above by mountingsemiconductor die 120 on the die mounting surface 444 of cavity 440 ofheader 430, making an electrical connection between the header and thelight-emitting surface of the die, and introducing a measured quantityof index matching material 154 into cavity 440. Threaded side surface415 of GRIN element 410 is then engaged with the threaded side surface446 of cavity 440 in header 430 and GRIN element 410 is rotated relativeto header 430 until the end surface 456 of body portion 452 abuts thesurface 434 of header 430. The juxtaposition of end surface 456 andsurface 434 defines cavity 442 and additionally defines the spatialrelationship between the light-emitting surface of semiconductor die 120and the end surface 416 of GRIN element 410. In one embodiment, anadhesive (not shown) is applied to either or both of threaded sidesurface 415 and threaded surface 446 prior to engaging the threads.After curing, the adhesive holds GRIN element 410 in place in header 430and prevents contaminants from leaking past the threads andcontaminating semiconductor die 120. LED 400 is then ready for finalelectrical and electro-optical testing.

GRIN element 410 is manufactured by processes similar to those describedabove for forming GRIN element 210 with the exception that the moldingprocess defines body portions 450 and 452, and additionally defines thethreaded side surface 415 of body portion 450. GRIN elements 410 withdifferent length-to-pitch ratios can be provided in ways similar tothose described above with reference to GRIN element 210.

FIGS. 8A–8C are respectively an exploded side view, a top view and across-sectional view of another embodiment 500 of an LED in accordancewith the invention. In this embodiment, the header has a threaded sidesurface that engages with the threaded side surface of a cylindricalcavity defined in the GRIN element. LED 500 is composed of a GRINelement 510, semiconductor die 120 and a cup-type header 530.

Header 530 has a device mounting surface 532, a surface 534 opposite thedevice mounting surface, and a threaded, curved side surface 560. LED500 is mounted on a printed circuit board or other substrate (not shown)with device mounting surface 532 facing the printed circuit board.Device mounting surface 532 carries conductive elements (not shown tosimplify the drawing) electrically connected to die 120.

Header 530 defines a cavity 540 that extends into the header from headersurface 534. Cavity 540 is a closed cavity that has a flat die mountingsurface 544 remote from header surface 534. Cavity 540 additionally hasa side surface 546. In the example shown, cavity 540 is cylindrical inshape and side surface 546 is cylindrical. Cavity 540 may be differentin shape from the example shown. Cavity 540 accommodates semiconductordie 120 mounted on die mounting surface 544. Cavity 540 additionallyaccommodates an electrical connection and above-described index matchingmaterial 154. The electrical connection is typically a wire bond andextends between the exposed surface of the die and an electricalconductor that extends through header 530 to device mounting surface532. Neither the electrical connection nor the electrical conductor isshown to simplify the drawing.

GRIN element 510 has a body 512 that exhibits the cylindrical refractiveindex profile described above. Body 512 is cylindrical in shape. Theaxes of symmetry of the refractive index profile and the body coincide.Body 512 has a curved side surface 514 and opposed end surfaces 516 and518. End surface 516 is annular, flat and orthogonal to theabove-mentioned axes of symmetry. In the example shown, end surface 518is circular, flat and orthogonal to the above-mentioned axes ofsymmetry. In other embodiments, at least end surface 518 is curved, asdescribed above. Body 512 may have shapes other than cylindrical, butexhibits a cylindrical refractive index profile regardless of its shape.

The body 512 of GRIN element 510 defines a cavity 542 that extends intobody 512 from the end surface 516. Cavity 542 is a closed cylindricalcavity and has a flat, circular end surface 562 remote from end surface516. Cavity 542 additionally has a curved, threaded side surface 564structured to engage with the threaded curved side surface 560 of header530.

The length of GRIN element 510 depends on the pitch of the cylindricalrefractive index profile of body 512 and the desired radiation pattern.As noted above, LEDs having many different radiation patterns can all bestructurally identical apart from the length-to-pitch ratio of the body512 of their GRIN elements 510. In this embodiment, the length of body512 is measured between end surfaces 562 and 518.

LED 500 is made in a manner similar to that described above by mountingsemiconductor die 120 on the die mounting surface 544 of cavity 540 inheader 530, making an electrical connection between the header and thelight-emitting surface of the die, and introducing a measured quantityof index matching material 154 into cavity 540 in the body 512 of GRINelement 510. Threaded side surface 560 of header 530 is then engagedwith the threaded side surface 564 of cavity 540 in GRIN element 510 andthe header is rotated relative to the GRIN element until header surface534 abuts the end surface 562 of cavity 542. The juxtaposition of headersurface 534 and end surface 562 defines the spatial relationship betweenthe light-emitting surface of semiconductor die 120 and the end surface562 of cavity 542 in GRIN element 510. In one embodiment, an adhesive(not shown) is applied to either or both of the threaded side surface564 of cavity 542 and the threaded side surface 560 of header 530 priorto engaging the threads. After curing, the adhesive holds header 530 inplace in GRIN element 510 and prevents contaminants from leaking pastthe threads and contaminating semiconductor die 120. LED 500 is thenready for final electrical and electro-optical testing.

GRIN element 510 is manufactured by processes similar to those describedabove for forming GRIN element 210 with the exception that the moldingprocess that defines the basic shape of the body 512 additionallydefines cavity 542 that extends into body 512 from end surface 516 andthe threaded side surface 564 of cavity 542. GRIN elements 510 withdifferent length-to-pitch ratios can be provided in ways similar tothose described above with reference to GRIN element 210.

FIG. 9 is a cross-sectional view of another embodiment 600 of an LED inaccordance with the invention. In this embodiment, the GRIN elementdefines a single cavity and no cavity is defined in the header. Elementsof LED 600 that correspond to elements of LED 500 described above withreference to FIGS. 8A–8C are indicted using the same reference numeralsand will not be described in detail again. LED 600 is composed of a GRINelement 510, semiconductor die 120 and a header 630.

Header 630 has a device mounting surface 632, a die mounting surface 634opposite the device mounting surface, a threaded, curved side surface660, and a flange 670 that extends out from side surface 660. Flange 660has an annular flange surface 672 opposite device mounting surface 632.LED 600 is mounted on a printed circuit board or other substrate (notshown) with device mounting surface 632 facing the printed circuitboard. Device mounting surface 632 carries conductive elements (notshown) electrically connected to die 120.

Semiconductor die 120 is mounted on the die mounting surface 634 ofheader 630.

The body 512 of GRIN element 510 defines a cavity 542 with a curved,threaded side surface 564 whose threads are structured to engage withthe threaded curved side surface 660 of header 630, as described above.The depth of cavity 542 is greater than the dimension of header 630 fromdie mounting surface 634 to flange surface 672. As a result, when LED600 is assembled, header 630 occupies only part of cavity 542 to definea cavity 640, i.e., the portion of cavity 542 not occupied by header630. Cavity 640 accommodates semiconductor die 120, an electricalconnection, and above-described index matching material 154. Theelectrical connection is typically a wire bond and extends between theexposed surface of the die and an electrical conductor that extendsthrough header 630 to mounting surface 632. Neither the electricalconnection nor the electrical conductor is shown to simplify thedrawing.

LED 600 is made in a manner similar to that described above by mountingsemiconductor die 120 on the die mounting surface 634 of header 630,making an electrical connection between the header and thelight-emitting surface of the die and introducing a measured quantity ofindex matching material 154 into cavity 542 in GRIN element 510.Threaded side surface 660 of header 630 is then engaged with thethreaded side surface 564 of cavity 542 in GRIN element 510 and theheader is rotated relative to the GRIN element until flange surface 672abuts the end surface 516 of GRIN element 510. The juxtaposition offlange surface 672 and end surface 516 defines the cavity 540 andadditionally defines the spatial relationship between the light-emittingsurface of semiconductor die 120 and the end surface 562 of cavity 542in GRIN element 510. In one embodiment, an adhesive (not shown) isapplied to either or both of threaded side surface 564 and threaded sidesurface 660 prior to engaging the threads. After curing, the adhesiveholds header 630 in place in GRIN element 510 and prevents contaminantsfrom leaking past the threads and contaminating semiconductor die 120.LED 600 is then ready for final electrical and electro-optical testing.

This disclosure describes the invention in detail using illustrativeembodiments. However, it is to be understood that the invention definedby the appended claims is not limited to the precise embodimentsdescribed.

1. A light-emitting device, comprising: a semiconductor light-emittingdie; a gradient index (GRIN) element having a cylindrical refractiveindex profile in which the refractive index varies radially and issubstantially constant axially, the GRIN element comprising a first endsurface opposite a second end surface and further comprising acylindrical body having a first diameter, the cylindrical bodycharacterized by a length-to-pitch ratio, the GRIN element arranged withthe first end surface adjacent the light-emitting die to receive lighttherefrom and emitting the light from the second end surface in aradiation pattern dependent on the length-to-pitch ratio; a headercomprising a cavity extending thereinto, the cavity having a seconddiameter slightly smaller than the first diameter, the light-emittingdie mounted in the cavity defined by the header; and a push fit hermeticseal defined by an assembly comprising the GRIN element engaged in thecavity.
 2. The light-emitting device of claim 1, in which: thelength-to-pitch ratio is equal to one fourth; and the GRIN element emitsthe light in a collimated beam.
 3. The light-emitting device of claim 1,in which: the length-to-pitch ratio is less than one fourth; and theGRIN element emits the light in a diverging beam.
 4. The light-emittingdevice of claim 1, in which: the length-to-pitch ratio is between onefourth and one half; and the GRIN element emits the light in aconverging beam.
 5. The light-emitting device of claim 1, in which: thelength-to-pitch ratio is equal to one half; and the GRIN element emitsthe light from a point source.
 6. The light-emitting device of claim 1,additionally comprising index matching material located in the cavity.7. The light-emitting device of claim 1, in which the second diameter issmaller than the first diameter in a range of 30–40 μm.
 8. A method ofmaking a light emitting device, the method comprising: providing asemiconductor light-emitting die; providing a GRIN element having acylindrical refractive index profile in which the refractive indexvaries radially and is substantially constant axially, the GRIN elementcomprising a first end surface opposite a second end surface and furthercomprising a cylindrical body having a first diameter, the cylindricalbody characterized by a length-to-pitch ratio; arranging the GRINelement with the first end surface thereof adjacent the light-emittingdie to receive light therefrom, the GRIN element emitting the light fromthe second end surface in a radiation pattern that depends on thelength-to-pitch ratio; providing a header comprising a cavity extendingthereinto, the cavity having a second diameter slightly smaller than thefirst diameter, the light-emitting die mounted in the cavity defined bythe header; and providing a push fit hermetic seal defined by anassembly comprising the GRIN element and the cavity, the push fithermetic seal being formed by engaging the GRIN element with the cavity.9. The method of claim 8, in which: providing a GRIN element comprises:providing GRIN elements each having a cylindrical refractive indexprofile, comprising a first end surface opposite a second end surface,and characterized by respective length-to-pitch ratio, thelength-to-pitch ratios differing among the GRIN elements, and selectingone of the GRIN elements as a selected GRIN element, the selected GRINelement having a length-to-pitch ratio corresponding to a desiredradiation pattern; and in the arranging, the selected GRIN element isarranged with the first end surface thereof adjacent the light source.10. The method of claim 8, in which providing a GRIN element comprises:providing an elongate rod having a cylindrical refractive index profilecharacterized by a pitch; and dividing off from the rod a lengthwiseportion to provide the GRIN element.
 11. The method of claim 8,additionally comprising dividing the rod lengthwise into portions, onesof the portions having different lengths to provide the GRIN elements oflight-emitting devices having different radiation patterns.
 12. Themethod of claim 8, additionally comprising: providing an additional GRINelement having a cylindrical refractive index profile, comprising afirst end surface opposite a second end surface and characterized by alength-to-pitch ratio different from the length-to-pitch ratio of theGRIN element; and substituting the additional GRIN element for the GRINelement to change the radiation pattern of the light-emitting device.